This invention relates to chemically coated fasteners having improved ease of penetration into a substrate, and improved resistance to withdrawal from the substrate. This invention also includes a process for preparing the chemically coated fasteners which is integral with a heat treating and hardening process, and does not require separate cleaning of the fasteners.
Chemical coatings on objects have been employed for the purposes of protecting the objects from corrosion and imparting aesthetic properties. The coated objects can be made from metals such as steel, iron, aluminum, or the like, or from other materials such as wood, plastic or paper. U.S. Pat. No. 5,283,280, issued to Whyte, discloses a process in which an aqueous bath containing a polymer solution is heated to about 80-160xc2x0 F. A metal object is heated to about 220-1700xc2x0 F. and immersed in the bath, causing a polymer coating to form on the surface of the object. Examples of suitable polymer solutions include those containing water-reducible alkyd resins, acrylic polymers, urethanes, multi-functional carbodiimides, melamine formaldehyde resins, styrene-acrylic copolymers, and polyolefin waxes.
Aqueous polymer coatings useful for coating objects are also disclosed in U.S. Pat. Nos. 5,458,659; 5,605,722; 5,605,952; 5,605,953; and 5,609,965; all of which are issued to Esser. All of these patents are directed to providing protective coatings or aesthetic finishes on various substrates.
In the construction industry, there is always a need or desire for fasteners having easier penetration into wood, metal and other substrates. Fasteners such as nails, for instance, are typically driven into substrates using nail guns and other power tools. Fasteners which penetrate substrates more easily reduce the energy, and often the weight, required for a power driving tool.
There is also a need and desire in the construction industry for nails and other fasteners which remain embedded in the substrate, and do not easily retract or withdraw from the substrate. Unfortunately, fasteners which are relatively easy to drive into a substrate often tend to withdraw more easily. Fasteners which are more difficult to withdraw also tend to be more difficult to drive into the substrate. Thus, it has been relatively difficult to develop fasteners which have excellent ease of penetration as well as strong resistance to withdrawal.
The present invention is directed to an elongated fastener which has improved ease of drive into a substrate, as well as improved withdrawal resistance, and a method for coating the improved fastener which can be integrated with an in-line heat treatment and hardening process.
In accordance with the invention, an elongated metal fastener (for example, a nail) is first heated to a temperature of about 500-2000xc2x0 F. An aqueous solution or mixture is prepared containing about 5-99% by weight water and about 1-95% by weight of an acrylic or modified acrylic polymer or copolymer, and/or a derivative thereof. The heated fastener is then quenched (i.e. rapidly cooled) by immersing it in the aqueous polymer solution or mixture. It is not necessary to clean the fastener (to remove oils, etc.) before heating and quenching it.
The rapid cooling of the fastener causes the formation of a substantially uniform and stable coating of the polymer on the fastener surface. When the fastener is heated to the above temperature range before quenching, and quenched using the described polymer bath, the resulting polymer coated fastener exhibits surprising and unexpected properties. Specifically, the coated fastener exhibits a combination of improved ease of drive into a wood substrate, and improved resistance to withdrawal, compared to a similar but uncoated fastener. Because no cleaning of the fastener is required, the polymer coating step can be part of an in-line process during which the fasteners are partially or totally heat treated, to effect hardening.
The coated fasteners of the invention exhibit these improved properties regardless of whether or not the fasteners are washed prior to coating. Also, the chemical coating does not inhibit the resistance welding of fasteners to one or more collation wires, to form a collated fastener assembly. This may be due to the fact that the resulting chemical coating is very thin or discontinuous (e.g. cracked) so that the electrical conductivity through the coating is not compromised.